The present invention relates generally to the machining of steels with diamond tools, and more particularly, to a method of substantially reducing the wear rate of diamond tools during the machining of steel workpieces. The present invention was made as a result of work under Contract W-7405-ENG-26 with the United States Department of Energy.
Diamond machine tools have been used with success for providing soft materials such as copper or ceramics with optical quality surface finishes with highly accurate contours. The use of the diamond tools for machining such metals with such surface finishes is that relatively ductile metals such as copper and some brittle ceramic materials possess low chemical activity with carbon, low yield strengths, low toughness, and chips are formed on these materials with a relatively low energy input due to the ductility of the copper and brittle fracture mechanism of ceramics. Efforts to provide workpieces of steel or similarly hard alloys with optical quality surfaces and accurate dimensions by using diamond cutting tools have not proven to be successful because the wear rate of the diamond cutting tools is extremely excessive. In fact, it has been reported that the wear rate for diamond cutting tools used for machining hardened steel is about 10,000 times greater than that for similar diamond tools used for machining brass. The wear rate of diamond tooling for machining optical quality surfaces on hardened steel is such that only a surface area of less than about 0.3 square inch can be finished with the tool before the tool needs to be replaced. Steel is characterized by possessing toughness, strength, chemical reactivity with carbon.
Efforts to determine the cause or causes for the wear rate of diamond tools and to overcome this problem so that machining of steel and other hard metals can be achieved with diamond tooling in order to provide desired optical quality surface finishes have not proven to be successful. Using expert machining practices with precision equipment did not lessen the wear rate of the diamond tooling to an acceptable level. Investigations indicated that the excessive wear rate on the diamond cutting tool when machining hardened steel was not caused by overheating, abrasion, or mechanical failure by microclevage which are usually the mechanisms responsible for the failure of conventional cutting tools. It appears that the failure of the diamond cutting tool is due to the conversion of the surface layer of the diamond tool in contact with the steel from the diamond form of carbon to a graphite carbon form with the constituents in the steel such as iron or nickel acting as a catalyst for this conversion.